In the art of gas chromatography, the process of thermal modulation has made possible comprehensive multi-dimensional separation techniques that have revealed startling complexity in many chemical mixtures, especially petroleum and petroleum-derived liquids.
In the prior art, thermal modulation methods have comprised the steps of heating sections of capillary columns with electrically pulsed resistive films, or the creation of heated or cooled zones moving in a longitudinal direction along segments of columns, the movement of the zones being mediated by mechanical devices. Resistive heating techniques, and mechanically swept heating techniques are described in U.S. Pat. No. 5,196,039 to Phillips et al, and U.S. Pat. No. 6,007,602 to Ledford, Jr. et al, and in European Patent Specification No. EPO 522 150 B1 corresponding to PCT publication WO 92/13622, all of which are incorporated herein in their entireties by reference. Longitudinally translated cooling techniques are described by Marriot in P. Marriot, and R. Kinghorn, Trends in Analytical Chemistry, 1999, 18, 114, which is also incorporated herein in its entirety by reference.
Mechanical translation techniques have the advantage of being more robust and reliable than resistive heating techniques. Mechanical translation techniques employed in the prior art have certain disadvantages however. In general, moving heaters or coolers are undesirable in that they make for complex apparatus prone to various forms of mechanical failure. The positioning of columns in mechanically translated heaters and coolers is inconvenient. The inertia of mechanically translated heaters and coolers sets limits on the frequency of thermal modulation. This is a severe limitation, since higher dimensional chromatographic techniques, such as three-dimensional gas chromatography, benefit from high frequency thermal modulation. Prior art embodiments also have employed the ambient stirred oven bath of the gas chromatograph to heat or cool sections of the modulator tube. The heating and cooling rates derived from the stirred oven bath set limits on minimum achievable chemical pulse widths, hence the frequency of thermal modulation.
The art of thermal modulation is considerably improved by the method of the present invention wherein heating and cooling segments of modulator tubes achieve modulation frequencies in the range of 2 Hz to 20 Hz. The method entails no moving parts in the vicinity of the modulator tube and does not present difficulties with respect to aligning the modulator tube with heating and cooling means.
The present invention provides an apparatus and method of thermal modulation that includes directing jets of gas flowing substantially perpendicular to a modulator tube in a chromatographic separation device, and preferably comprises directing pulsed jets of gas perpendicular to a gas chromatographic modulator tube. The apparatus of the present invention provides means to direct jets of gas substantially perpendicular to a modulator tube and preferably provides means to supply pulsed jets of gas. A surprising result is that even within the stirred oven bath of a gas chromatograph, the jets can heat and cool segments of a modulator tube at least 2.0 cm away from a nozzle exit orifice to temperatures, and at thermal heating and cooling rates, suitable for high speed thermal modulation. The ambient oven bath, even though it is strongly stirred by means of a fan inside the GC oven, does not interfere with the cooling or heating action of the gas jets directed onto the modulator tube. Because gas jets are spatially diffuse, and nozzles may be physically distant from the modulator tube, the act of mounting the modulator tube in the path of the gas jets is far more straightforward than prior art techniques for aligning modulator tubes with mechanically rotated or translated heating and cooling means, an important matter from the standpoint of ease of use and commercialization.
Herein, the present method of thermal modulation by means of gas jets directed substantially perpendicular to a modulator tube in a chromatographic apparatus are referred to as methods of xe2x80x9ctransverse thermal modulationxe2x80x9d, or more simply, xe2x80x9ctransverse modulation.xe2x80x9d The advantages of transverse thermal modulation will become more apparent in view of a detailed description of the method with reference to FIG. 1, which represents one embodiment of the present invention as it would be employed in a comprehensive two-dimensional gas chromatograph.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.